Over the years, optical fiber transmission systems have increased in capacity from about 45 megabits per second (Mb/s) to 1.7 gigabits per second (Gb/s). Failure to operate at a guaranteed level of bit-error rate (BER) depends in part on connections along an optical path and the strength of received signals. BER degradation has been attributed to reflections in the optical path. A series of reflection points can generate multiple reflections among themselves thereby worsening the degradation. This is particularly important in high speed lightwave transmission systems (over 1.0 Gb/s) and amplitude modulated (AM) cable television (CATV).
A very much used connector for terminating and connecting two optical fibers is one which is referred to as a biconic connector. The biconic connector is disclosed in U.S. Pat. No. 4,512,630 which issued on Apr. 23, 1985 in the name of P. K. Runge and in an article authored by T. L. Williford, Jr., K. W. Jackson and C. Scholly appearing at page 87 in the Winter 1980 issue of the Western Electric Engineer and entitled "Interconnection for Lightguide Fibers".
The biconic connector may include two plugs each having a cylindrical portion and a truncated, conically shaped portion with a passageway extending therethrough. An optical fiber end portion is received in the passageway and extends to an end of the truncated conically shaped portion. Each plug is received in one of two conically shaped cavities of a sleeve with surfaces of walls which define the cavities having surfaces which are conformable with those of the truncated, conically shaped portions of the plugs. When the plugs are seated in the sleeve, the end portions of the plugs become disposed adjacent to each other.
Connections between optical fiber ends require great care. Because the core diameter of the optical fiber may be as small as 8 .mu.m, it is difficult to align precisely cores of two optical fibers to be connected to achieve tolerable losses. Not only do the cores of the end portions of two optical fibers to be connected need to be aligned, but also the axes of the optical fiber end portions must be parallel.
Attenuators often are required in an optical fiber transmission path to reduce the strength of an incoming signal to a desired level. Often times, the required attenuation is induced at a patch panel or at an optical backplane where it is most convenient to insert an attenuator between connectors.
Many optical fiber communication systems require a method of decreasing optical power at a reducing station to avoid the saturation of receivers. Such a reduction in power may be accomplished by introducing into the system a device which is designed to dissipate or to attenuate a controlled fraction of the input power while allowing the balance to continue through the system.
Changing attenuation level also may be required. It is known that the efficiency of a circuit decreases with age and that the power of a signal source which may be adequate at the beginning of life of a circuit later may become inadequate. If the power of the signal at the beginning is chosen so that it remains adequate later, components of the circuit may become saturated early in life. Additionally, the unearthing of cable which results in repairs that add optical loss to the transmission path can be compensated for with a lower loss attenuator.
Such attenuators are available commercially in various configurations. Some are inserted at patch panels as mentioned hereinbefore and some are in-line such as is shown in U.S. Pat. Nos. 4,257,671 and 4,261,640. Typically, prior art in-line attenuators are noncontacting, that is, they are not contacted by the fiber ends in the biconic connector plugs.
In one commercially available attenuator, a removable, low loss, low cost frustoconical shaped shim or sleeve of predetermined wall thickness is used in combination with a biconic connector. See U.S. Pat. No. 4,714,317. The shim can easily be removed or replaced with another. Hence, an incoming line may be easily converted to a low loss situation and then converted back or changed to another predetermined loss.
Also of interest is W. C. Young U.S. Pat. No. 4,213,932 which issued on July 22, 1980 and in which a biconical socket is shown to include an internal seat. The seat may be used to position a septum, which can be fabricated from a translucent material to introduce attenuation into an optical path. Alternatively, the septum can be a filter or a wafer soaked in an index matching fluid to improve the optical transmission.
The use of an attenuator in an optical path raises a concern about high reflections and reflected power for systems operating above 1.0 Gb/s. High bit rate systems have been plagued by high reflective loss from attenuators that vary either in the length of the air gap or in carbon density. Reflections in the optical path have not been an issue for systems operating at less than 1.0 Gb/s.
Typically, fixed air gap, non-plug contacting or high density filter elements have beed used in optical transmission systems and data links that use multimode-to-multimode or single mode-to-multimode connections. Typically, these systems of less than one gigabit per second are not affected adversely by high reflected power. However, there is a need for a low reflection attenuator that can be used in single mode-to-single mode connections in optical systems that operate above 1.0 Gb/s.
Light which is reflected from components such as connectors and splices along a fiber link can strike a source of light such as a laser, for example, which may affect adversely the performance of the laser. Optical power fluctuation, pulse distortion and phase noise may result. Also affected adversely may be the wavelength, linewidth and threshold current of the laser.
Multiple reflections from two or more connections may cause system degradation which is referred to as multiple path intereference (MPI). MPI is a phenomenon well known in classical optics and is realized whenever there are two or more optical discontinuities. The two major mechanisms that cause optical discontinuities are connections which are less than ideal and air gap attenuators.
Reflections reduce the signal-to-noise ratio of a receiver by two effects. First, multiple patterns from interferometric cavities that feed back into the transmitter can cause a conversion of the laser's phase noise into intensity noise. The receiver picks up the degraded signal. Also, multiple paths can introduce spacious "ghost signals", which arrive at the detector within variable delays, thereby producing intersymbol interference. Both effects result in an effective power penalty of several dB at the receiver. Inasmuch as these effects are signal dependent, increasing the transmitted power does not improve the error performance. Bit-error ratio floors have been observed in laboratory gigabit/second fiber transmission systems due to multiple reflections from connectors and splices.
Reflections occur at a glass-air interface because of the difference in the refractive indices of the two materials. Each optical fiber with its end face cleaved perpendicularly to the fiber axis reflects at about a 3.5% level. When optical fiber ends are polished, the refractive index increases for a thin surface layer whereupon the reflectance can increase to over 5.5%.
Two surfaces such as the end surfaces of two spliced optical fibers form a cavity within which multiple reflections can occur. When the distance between the end faces equals an integral number of half wavelengths of the transmitter wavelength, all round trip distances equal an integral number of in-phase wavelengths and constructive interference occurs. This cause a quadrupling of reflectance of about 14% for unpolished end faces and to over 22% for polished end faces. On the other hand, a quarter wavelength displacement of the surfaces leads to constructive interference and no reflection.
One way of reducing reflective effects at a transmitter is to use an optical isolator which prevents light from reentering the laser. However, the use of an isolator results in some additional forward transmission loss and possible polarization effects.
Reflectances of components also can be reduced by using an index matching oil or gel between interfaces. Perfect matching is not likely because of the difficulty in matching the complex refractive index profile of the optical fiber, attraction of airborne dust, and because of temperature effects on the index material. Connectors which provide for contacting end faces can be used, if care is taken not to damage the end faces during installation or service. Another prior art technique has been to prepare optical fiber end faces at an angle or with a curved surface so that reflected light is directed away from the optical fiber axis and does not reenter either of the connected fibers. However, angled connectors may result in a slightly increased transmission loss and require both connector plugs to be replaced. Also, anti-reflective coatings can be applied to ends of fibers, but both plugs must be coated, requiring replacement of existing plugs in pairs.
Clearly, what is needed and what has not been provided by the prior art is an in-line, low reflection attenuator which overcomes the foregoing problems. The sought-after attenuator is required for high speed lightwave transmission systems with distributed feedback lasers, and amplitude modulated cable television transmission where unwanted reflections in the network can result in optical feedback into the laser causing laser instability and receiver noise. Also, the sought-after low reflection attenuator is needed to minimize systems degradations due to multiple path interference. The sought after in-line attenuator must be structured keeping in mind that the level of reflected power can be affected by a mismatch in index of refraction in the transmission path, by the length of the gap between optical fiber ends, by laser linewidth, by frequency and by the distance between the two connections.
What is sought and what does not appear to be available in the prior art is a biconic connector which includes an in-line attenuator which results in low return loss. Desirably, the sought after attenuator may be integrated easily and be compatible with existing biconic connection systems.